Computational Methods of Scientific Programming

Transcription

1 Computational Methods of Scientific Programming Lecturers Thomas A Herring, Jim Elliot, Chris Hill, Summary of Today s class We will look at Matlab: History Getting help Variable definitions and usage Math operators Control statements: Syntax is available through the online help M-files: Script and function types Variable number of input and output arguments Our approach here will be to focus on some specific problems using Matlab for analysis and for building Graphical User Interfaces (GUI) and treating graphics as objects. 10/31/ Lec

2 MATLAB (Matrix Laboratory) History MATLAB was originally written to provide easy access to matrix software developed by the LINPACK and EISPACK projects. First version was released Current version is version 6 (specifally 6.5 is available on all platforms) Interactive system whose basic data element is an array that does not require dimensioning UNIX, PC and Mac versions. Similar but differences. 10/31/ Lec 16 3 MATLAB: All commands are executable although there is the equivalent to dimensioning. In general arrays in MATLAB are not fixed dimensions Syntax is flexible but there are specific set of separators Basic Structure: MATLAB commands are executed in the command window called the base workspace (>> prompt) MATLAB code can be put in M-files: Two types Script type which simply executes the code in the M-file Function type which executes codes in a new workspace. Generally variables in the new workspace are not available in the base workspace or other workspaces. 10/31/ Lec

3 Getting help Matlab has extensive help available both locally based and through the web. In release 13 there is a help menu in the command window. Help falls into two types: Help on specific commands and their usage Help by topic area which is useful when looking for generic capabilities of Matlab Matlab also comes with guides and there are thirdparty books such as Mastering Matlab 5 10/31/ Lec 16 5 Basic Structure 02 Variable types Variables are either double precision, strings cells {}, or structures. Complex variables are used as needed (use *i or *j to set complex part) Variables can be defined locally in current workspace or they can be global. To be global must be defined that way in both base workspace and M-files who and whos are used determine current workspace variables Names are case sensitive, no spaces, start with letter and may contain numbers and _ workspace command is GUI management tool (now built into Desktop Layout). 10/31/ Lec

4 Basic Structure 03 I/O: File I/O is similar to C fopen,fclose, fread (binary), fwrite (binary), fscanf (formatted read), fprintf (format write), fgetl (read line), fgets (read line keep new line character), sscanf (string read), sprintf (string write) save and load save and load workspace. Math symbols: + - * / \ ^ (\ is left divide) When matrices are used the symbols are applied to the matrices. When symbol preceded by. Array elements are operated on. means transpose array or matrix Lec01_01 and Lec01_02 are examples 10/31/ Lec 16 7 Basic Structure 04 Control if statement (various forms) for statement (looping control, various forms (similar to do) while statement (similar to do while) No goto statement! break exists from for and while loops switch case otherwise end combination try catch end combination Termination end is used to end control statements above return is used in functions in the same way as Fortran. 10/31/ Lec

5 M-files: Script and Function types Communication with functions and M-files Script M-files: Do not accept input or output arguments Operate on data in workspace Useful for automating a series of steps Function M-files Accept input arguments and return outputs Internal variables are local to the function by default, but can be declared global Useful for extending language 10/31/ Lec 16 9 Syntax Flexible layout with certain characters have specific uses. % is the comment symbol. Everything after % is ignored (3 dots) is the line continuation symbol. Must be used at a natural break in commands, used to separate commands, with result printed ; used to separate commands with result not printed [] enclose arrays and matrices, {} enclose sets (difference is multidimensional arrays need to be all of the same type and size) : is the range selector for from start:increment:end, if only one : increment is 1, if no numeric values, range for matrix elements. 10/31/ Lec

6 Multidimensional arrays Matlab works naturally with 1 and 2 dimensional arrays but more than 2 dimensions can be used. They can be constructed a number of different ways By extension: a = [5 7 8 ; ; 4 3 6]; a(:,:,2) = [ ; 3 5 6; 9 8 7] Scalar extension (Set plane 3 to 5) a(:,:,3) =5 Use of functions ones, zeros, randn b = zeros(3,3,2) cat function, cat(ndim, arrays, ) where ndim is the dimension to be concatinated in. 10/31/ Lec Multidimensional arrays 02 reshape function allows refinition of array shape e.g., a = [1:18]; reshape(a,[ ]) squeeze removes dimensions that are only 1 element permute allows array dimensions to be re-ordered. Functions that operate on elements of arrays work with multidimensional arrays but matrix type functions do not work unless a suitable 2-D array is passed Functions that operate on vectors use the first nonsingleton index 10/31/ Lec

7 Multidimensional cells and structures Cell arrays are similar to multidimensional arrays except that the all the cells do not need to be same e.g., a{1,1} = [ 1 2 ; 4 5]; a{1,2} = Name ; a{2,1} = 2-4i; Structure arrays also exist and are accessed and created similar to C (i.e., elements are referred to by. construction patient.name = John Doe ; patient.age = 32; These are recent features added to Matlab and can be useful in many applications but we will not discuss further. 10/31/ Lec Program Layout Matlab can be run interactively; with script M-files as we have been doing; and/or function M-files It is possible to execute C-compiled routines called MEX files (for speed) but we will not cover this (system dependent) PC Matlab supports Word Notebooks but not available on Unix. Status on Mac is not clear yet. helpwin on all systems invokes the help system tour and demo give a tour and demo of Matlab 10/31/ Lec

8 Function M-files Function M-files can have multiple inputs and outputs The generic construction is (in an M-file whose name is that of the function.m) function y = flipud(x) % FLIPUD Flip a matrix up/down % Comments about function.. Actual code Name must begin with a letter First line is function declaration line First set of contiguous comment lines are for help First comment (H1 line) is searched with the lookfor command 10/31/ Lec Function M-files 02 Usually name is capitalized in H1 line Functions can invoke M-file scripts (executed in function workspace) M-file can contain multiple functions that are subfunctions of main function in mfile Functions can have zero inputs and outputs nargin tells number of arguments passed in call nargout tells how many outputs given Normally input variables are not copied to function workspace but made readable. However, if there values are changed then they are copied 10/31/ Lec

9 Function M-files 03 Functions can accept variable and unlimited numbers of input variables by using varargin as the last argument Functions can have variable numbers of outputs used varargout. Use the command global to have variables shared between base workspace and function workspace (must be declared global in both places). Matlab lets you reach another workspace with the evalin function You can also use assignin to assign values in a workshop (not recommended) 10/31/ Lec Summary of Introduction to Matlab Looked at the basic features of Matlab: Getting help Variable definitions and usage Math operators Control statements: Syntax is available through the online help M-files: Script and function types Variable number of input and output arguments 10/31/ Lec